MODELING OF SEISMIC-WAVE PROPAGATION NEAR THE EARTHS SURFACE

Realistic modeling of seismic wave propagation in the vicinity of the earth's surface is complicated by large velocity contrasts, strong het erogeneity, severe attenuation, and topographic relief. To account for these complications, we employ a finite-difference solution of the 2D viscoelastic equations, a grid-refinement technique iii the shallow p arts of the model, and a generalized imaging condition to model free-s urface topography. The grid-refinement approach allows us to vary the discretization of the model and the wavefield with respect to the velo city structure:. Compared to a standard uniform finite-difference grid approach, this saves considerable amount of memory and computations; thus enables modeling of wave propagation through large portions of th e earth's crust. Moreover, the decreasing accuracy of the finite-diffe rence method near the irregular free surface is compensated by using a finer grid-spacing in this region. Numerical tests show that the meth od is reliable and accurate. By applying this modeling technique to se veral canonical models of the near-surface region and upper crust, we find that scattering and mode conversions from topographic relief, wav eguide effects, and attenuation in the immediate subsurface tend to do minate the seismic coda at near lapse times. (C) 1997 Elsevier Science B.V.